Saturn moon looks like a sponge

As much as 40% of Hyperion could be porous, researchers say (Image: Ciclops)

Astronomers looking at Hyperion, a large Saturn moon, say the spud-shaped rock has a "sponge-like appearance" unlike anything ever seen.

As much as 40% of Hyperion could be porous, according to research led by Dr Peter Thomas of the Center for Radiophysics and Space Research at Cornell University published today in the journal Nature.

The US space probe Cassini sent back high-resolution images of Hyperion to Earth in four flybys in 2005 and 2006.

In its closest pass, it swooped to within 618 kilometres of its surface, getting picture resolution as fine as a few kilometres across.

"The most striking visual aspect of Hyperion is its sponge-like appearance, which is unlike any other object imaged to date," they report.

Hyperion is peppered with craters, most of which are 2-10 kilometres across, and are well preserved.

The apparent reason for this is that the surface is so porous that an impacting space rock causes little ejecta, material blasted out of the collision site and is scattered all around.

The moon has a mean diameter of 270 kilometres, but its shape is so irregular that this can be 190-364 kilometres.

It is the eighth-largest satellite and the largest irregularly-shaped satellite of the 48 named Saturnian moons.

Hyperion's strange shape may have been caused by bombardment by meteors, which blew away part of its surface, some experts believe.

In a companion paper, Dr Dale Cruikshank of NASA's Ames Research Center and colleagues report that Hyperion's surface is quite reflective but the bottoms of the craters are quite dark, similar to those found on the Saturnian moons Phoebe and Iapetus.